1. Field of the Invention
The present invention relates generally to a detector for apparatus that determines the velocity of fluid flow in a pipe or tube using ultrasonic waves and, more particularly, to a flowmeter detector for use with pipes having small bore diameters and/or low flow rates.
2. Discussion of the Related Art
Ultrasonic flowmeters use acoustic waves or vibrations of a frequency greater than 20 kHz to detect the velocity of a fluid flowing within a pipe or tube. Depending on the design, they use either wetted or nonwetted transducers mounted on the pipe perimeter to couple ultrasonic energy with the fluid flowing in the pipe.
The velocity of the fluid flowing within the pipe is determined on the basis of a difference between the propagation velocity of the ultrasound propagating downstream and that of the ultrasound propagating upstream. These so-called transit-time meters, as the name implies, measure the difference in travel time between pulses simultaneously transmitted in the direction of, and against, the fluid flow. This type of meter, also called time of flight and time of travel, uses a pair of transducers, each capable of both transmitting and receiving the ultrasound waves. One of the pair of transducers is located upstream of the other and each transducer alternately transmits and receives bursts of ultrasonic energy. The difference in the transit times in the upstream vs. the downstream directions (TUxe2x88x92TD) measured over the same path is used to calculate the flow through the pipe:
V=Kxc2x7D/sin 2xcex8xc2x71/(T0xe2x88x92xcfx84)2 xcex94T
where:
V=mean velocity of flowing fluid
K=constant
D=i.d. of the pipe
xe2x8ax96=incident angle of ultrasonic burst waves
T0=zero flow transit time
xe2x80x83xcex94T=T2xe2x88x92T1
T1=transit time of burst waves from upstream transmitter to downstream receiver
T2=transit time of burst waves from downstream transmitter to upstream receiver
xcfx84=transit time of burst waves through pipewall or lining
This equation shows that the flow velocity is directly proportional to the measured difference between upstream and downstream transit times. Because the cross sectional area of the pipe is known, the product of that area and the flow velocity will provide a measure of volumetric flow. Such calculations are typically performed by microprocessor-based converters associated with the flowmeter apparatus of such devices.
Flowmeters are classified into two distinct measurement types based upon the positioning of the ultrasonic transducers. In the oblique measurement type, the propagation course of the applied ultrasonic waves is tilted typically at 45xc2x0 from a flow direction of the measured fluid. In the parallel measurement type, the propagation course of the ultrasonic waves is substantially parallel to the flow direction of the fluid.
The oblique measurement type finds advantage in that it can be used on a straight length of measuring pipe and, therefore is substantially free from potential pressure losses and relatively simple in its construction. However, when the measuring pipe is small in diameter, the propagation course of the ultrasonic wave within the fluid is also shortened in length, which reduces the measuring accuracy of the ultrasound propagation time and, therefore, the accuracy of the fluid flow measurement. Consequently, the oblique measurement type of flowmeter construction is not suitable in measuring flow rate in pipes having a small bore diameter and/or a low flow velocity.
Typically, parallel measurement type flowmeters require that the transducers be smaller than the inside diameter of the pipe in order to efficiently couple the acoustic energy into the medium being measured and not into the pipe wall. Pipes having a small cross-section diameter (here defined as those having a cross-section diameter roughly equal to the transducer diameter) have a portion of the acoustic energy coupled directly into the pipe wall. The burst signal then arrives at the receiving transducer having followed two separate paths: one through the medium being measured and the other through the pipe wall. The two arrival times may be similar for applications that measure liquids since velocity in the liquid is similar to velocity in the solid pipe wall; however, they may be very different when the medium being measured is a gas. Therefore, the cross section size of the ultrasonic transducer becomes a limiting factor in the parallel measurement type of flowmeter design.
In accordance to the present invention, there is provided a flowmeter detector for measuring the flow volume of a fluid medium. The flowmeter detector includes a measuring line having an interior bore extending from a first closed end to a second closed end. A fluid medium entry port is located adjacent the first closed end extending from the exterior of the measuring line into the interior bore. The fluid medium entry port conveys fluid medium from a source into the measuring line. The flowmeter detector further includes a fluid medium exit port located adjacent the second closed end, extending from the interior bore to the exterior of the measuring line. The exit port conveys the fluid medium from the measuring line back to the source. The entry and exit ports can be oriented either perpendicularly or obliquely to the longitudinal axis of the interior bore.
A first acoustic energy transducer is attached to the first closed end. The first transducer is arranged to alternatively transmit and receive pulsed acoustic energy. Similarly, a second acoustic energy transducer is attached to the second closed end. The second transducer is similarly arranged to alternatively transmit and receive pulsed acoustic energy. The first and second acoustic transducers have an overall dimension that is less then the cross-section diameter of the interior bore of the measuring line, thereby coupling all of transducer acoustic energy into the interior bore.
Each transducer can be mounted directly to the measuring line end wall or through an acoustic coupler. The acoustic coupler is arranged to focus the acoustic energy into a measuring line interior bore that has a cross-section diameter less than the overall dimension of an attached transducer.
A control circuit is connected to the first and second acoustic transducers. The control circuit is operated into a first mode placing the first and second transducers into a sender mode which simultaneously, from opposite ends of the measuring line, transmits pulsed acoustic energy into the interior bore and the fluid medium flowing between the entry and exit ports. Subsequently, the control circuit is operated into a second mode which places the first and second transducers into a receiver mode each receiving the others"" acoustic energy propagated through the interior bore and the fluid medium.
The acoustic energy received by the first and second transducers is passed to an evaluation circuit. The evaluation circuit calculates the flow volume of the fluid medium flowing in the measuring line by determining the difference in the travel times of the pulsed acoustic energy transmitted by the first and second acoustic transducers.
It is, therefore, an object of the present invention to provide a flowmeter detector for use with pipes having small bore diameters an/or low flow rates.
It is also an object of the present invention to provide a flowmeter detector that efficiently couples all of the acoustic energy from a transducer into the interior bore of a measuring line having a small bore diameter.